The present invention relates to a composite semiconductor device used in a print head used of an image forming apparatus such as an electrophotographic printer.
Conventional optical print heads are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2004-179646 or “Design of Optical Printer” published by Triceps under the editorship of Yoshihiro Takekida.
The former document (i.e., Japanese Laid-Open Patent Publication No. 2004-179646) discloses a thin-film light-emitting element. The light-emitting element is connected to an integrated circuit (including driving circuits) via a thin-film wiring layer using a printed wiring technique.
With such an arrangement, it becomes possible to obtain an LED (Light Emitting Diode) which includes small number of expensive compound semiconductor materials, and to enhance the yield rate and the tolerance to mechanical stress (by virtue of printed wiring technique).
FIGS. 34 and 35 are respectively a perspective view and a plan view showing the conventional LED unit 1. As shown in FIG. 34, the LED unit 1 includes a unit substrate 2 (such as a glass substrate or a ceramic substrate), LED chips 3 including a plurality of light-emitting portion 6 arranged on the unit substrate 2, driver IC chips 4 for controlling the light-emitting portions 6, and bonding wires 5 including metal wires electrically connecting the LED chips 3 and the driver IC chips 4. The LED chips 3 and the driver IC chips 4 have the thickness of approximately 30 μm, and are bonded onto the unit substrate 2 by means of die-bonding.
As shown in FIG. 35, each LED chip 3 includes the light-emitting portions 6 that emit lights and individual electrodes 7 for supplying power to the light emitting portions 6. Each individual electrode 7 has the size of, for example, 100 μm×100 μm, in order to provide a sufficient region for wire-bonding using Au wire. The pn-junction that forms a light-emitting region is at a distance from 3 μm to 5 μm from the surface of the unit substrate 2.
However, the conventional print head using the above described LED unit 1 has problems as described below.
For reliable wire-bonding between the LED chip 3 and the driver IC chip 4, it is necessary to provide wire-bonding pads on each LED chip 3 and the driver IC chip 4. The wire-bonding pads occupy large regions on the LED chip 3, and therefore the light-emitting region on the LED chip 3 decreases. Since the rate of the light-emitting region to the surface area of the LED chip 3 decreases, the structure of the LED chip 3 is inefficient in terms of efficiency in use of semiconductor materials. In other words, since it is necessary to provide regions for wire-bonding pads on the LED chip 3, it is difficult to miniaturize the LED chip 3 to thereby reduce material cost.
Further, the light-emitting region formed is at a distance of approximately from 3 μm to 5 μm from the surface of each LED chip 3, whereas the thickness of the LED chip 3 is approximately 300 μm. Therefore, it is difficult to efficiently use the materials of the LED chip 3 in the direction of the thickness thereof. Moreover, GaAs substrate (i.e., the base material of the LED chip 3) only functions as a supporting body that supports GaAsP epitaxial layer having a light-emitting function. Furthermore, in order to prevent the short-circuit between the bonding wires and the LED chip 3 during the wire-bonding operation, the thicknesses of the LED chip 3 and the driver IC chip 4 need to be almost the same. Therefore, it is difficult to reduce the thickness of the GaAs substrate and to thereby reduce the material cost, while ensuring the function as a supporting body and the region for wire-bonding.